Conventionally, a cascode-connected amplifier circuit, which has better high-frequency characteristics than does a grounded-emitter amplifier circuit, has been used in an amplifier circuit and a gain-variable amplifier circuit.
The input impedance of the grounded-emitter amplifier circuit can be considered to be equal to a value obtained when a base-collector resistor Rbc and a base-collector capacitor Cbc are connected in parallel to a base-emitter capacitor Cbe.
The capacitance of the base-collector capacitor Cbc is multiplied by (Av+1) due to the mirror effect, where Av is the amplification degree of a grounded-emitter transistor used in the grounded-emitter amplifier circuit, so that the capacitive component of the input impedance is increased. This causes deterioration of the high-frequency characteristics of the grounded-emitter amplifier circuit. On the other hand, in the cascode-connected amplifier circuit, the amplification degree Av becomes 0, so that there is no influence from the mirror effect. This makes it possible to obtain good frequency characteristics.
FIG. 14 is a circuit diagram showing an arrangement of a conventional cascode-connected amplifier circuit as shown in Japanese Unexamined Patent Publication No. 308634/1998 (Tokukaihei 10-308634; published on Nov. 17, 1998).
As shown in FIG. 14, a cascode-connected amplifier circuit 200 includes a cascode connection of a grounded-emitter bipolar transistor Q201 and a grounded-gate MOS transistor M202, and is arranged such that a voltage inputted to a base of the transistor Q201 is outputted from a drain of the MOS transistor M202.
Further, a gate of the MOS transistor M202 is connected to a bias power supply V202, and a power supply voltage Vcc is applied to the drain of the MOS transistor M202 via a load resistor R202.
Gain to be obtained by the cascode-connected amplifier circuit 200 is adjusted in the following manner: The amount of current flowing through the base of the transistor Q201 is changed, and the amount of collector current is changed in accordance with the change of the amount of the base current. Therefore, amplification operation of the cascode-connected amplifier circuit 200 can be stopped by reducing, in order that the base-emitter voltage of the transistor Q201 is less than a transistor's threshold voltage, the amount of the current flowing through the base of the transistor Q201.
Specifically, when the transistor Q201 is switched off, the potential of a collector of the transistor Q201 is increased. Moreover, the collector of the transistor Q201 is connected to a source of the MOS transistor M202, so that the collector of the transistor Q201 and the source of the MOS transistor M202 have the same potential.
Therefore, when the transistor Q201 is switched off, the potential of the source of the MOS transistor M202 is increased. Accordingly, the gate-source voltage of the MOS transistor 202 becomes less than a transistor's threshold voltage. Therefore, the MOS transistor M202 is switched off.
However, in the cascode-connected amplifier circuit 200, in cases where an input terminal of the transistor Q201 receives a large signal, the electric power of the signal causes the transistor Q201 to be temporarily on, so that the potential of the collector of the transistor Q201 is reduced. Accordingly, the gate-source voltage of the MOS transistor M202 becomes not less than a transistor's threshold voltage. Therefore, the MOS transistor M202 is switched on. This raises such a problem that: the cascode-connected amplifier circuit 200 is temporarily on when it is supposed to be off, so that sufficient isolation is not attained.
Further, for example, see a case of a gain-variable amplifier circuit including a plurality of cascode-connected amplifier circuits 200. In this case, even when operation of a transistor of a first cascode-connected amplifier circuit stage is stopped so that operation of the first cascode-connected amplifier circuit stage is stopped, a voltage is always applied to a gate of a MOS transistor of the first cascode-connected amplifier circuit stage. This may affect an operational state of a second cascode-connected amplifier circuit stage. The same holds true in cases where the first cascode-connected amplifier circuit stage is in an operating state and the second cascode-connected amplifier circuit stage is in a nonoperating state. Therefore, signal leakage occurs at each amplifier stage. This raises such a problem that gain suppression and linearity are deteriorated.
In order to solve such problems, a cascode-connected amplifier circuit disclosed, for example, in Japanese Unexamined Patent Publication No. 312016/2005 (Tokukai 2005-312016; published on Nov. 4, 2005) includes a bipolar transistor and/or a field-effect transistor, and a base or gate of each of the transistors is provided with a control circuit.
However, even when a grounded-emitter bipolar transistor is off, the cascode-connected amplifier circuit may suffer from the following problem: In cases where a control voltage to be applied to a base of the bipolar transistor (or to a gate of a grounded-source field-effect transistor) cannot be sufficiently reduced, or in cases where the base current of a grounded-base bipolar transistor or the gate voltage of a grounded-gate field-effect transistor cannot be sufficiently reduced, the bipolar transistor (or the field-effect transistor) is switched on for operation.